In addition to what John G suggests, this may have been caused by a loose fitting male banana plug where it inserts into the female end of the binding post. A loose connection here or inside at the brick terminals can cause the overheating seen in the photo. Checking that the Z-1000 circuit breaker is functioning correctly would also be a good idea. It should trip when greater than 6 Amps of current is flowing for a few minutes or less.